Generally speaking, an image forming apparatus of an electrophotographic type forms an image on a printing medium through charging, exposing, developing, transferring and fusing processes. For example, FIG. 1 illustrates the relevant portions of a conventional electrophotographic type color image forming apparatus of a so-called tandem type, which may typically include a plurality of image carrying bodies 10 on which visible images are formed with developer (not shown) of different colors, a transferring unit 20 operable to transfer the visible images respectively formed on the image carrying bodies 10 to a printing medium M and a fusing unit 30 fusing the transferred image onto the printing medium M by applying heat and pressure.
In an image forming apparatus of the configuration shown in FIG. 1, the transferring unit 20 includes a plurality of transferring rollers 21 facing respectively the image carrying bodies 10 and a transferring belt 25 arranged to pass between the image carrying bodies 10 and the transferring rollers 21 while transporting thereon the printing medium M. A transfer voltage of a polarity opposite to that of the voltage being applied to the image carrying bodies 10 may be applied to the transferring rollers 21, to thereby cause the toner particles on the image carrying bodies 10 to be transferred to the printing medium M.
Such transfer voltage is to some extent also applied to the printing medium M so that, when the printing medium M passes through the transfer nip formed between an image carrying body 10 and a transferring roller 21, a gap electric discharge (or sometime also referred to as the printing medium division electric discharge) occurs due to the resulting electric potential difference between the image carrying body 10 and the printing medium M at the region in the vicinity of the transferring nip, particularly in the region immediately upstream or downstream of the transferring nip as indicated by the area A in FIG. 1. Such gap electric discharges can lead to an image scattering.
To address the above described image scattering by the gap electric discharge, as, for example, illustrated in FIG. 2, an electrical charge removal device 40 may be employed. The electrical charge removal device 40 includes a bracket 41 for mounting the electrical charge removal device 40 to the main body frame of a image forming apparatus and a needle shape electrode 45 supported on the bracket 41. The needle shape electrode 45 removes electrical charges from the printing medium to reduce an electric potential difference between an image carrying body 10 and the printing medium, thereby reducing the likelihood of occurrences of gap electric discharges.
When such an electrical charge removal device 40 is employed, the positioning of the electrical charge removal device 40 in relation to a transferring roller 21 and the distance between an end of the needle shape electrode 45 and the printing medium influence the charge removal performance. That is, the proper positioning of the electrical charge removal device 40 may be needed for the optimum performance that sufficiently reduces the occurrences of the gap electric discharges. For example, an improved performance may be realized by positioning the end of the needle shape electrode 45 close to the transfer nip, preferably in a space B, i.e., inside the diameter of the transferring roller 21.
As the transferring unit 20 including the transferring roller 21, to which the transfer voltage is applied, ordinarily has a shorter useful operational life than the useful life of the image forming apparatus as a whole, the transferring unit 20 is typically provided to be replaceable. From the cost effectiveness standpoint, it is more advantageous to be able to replace only the transferring roller 21 along with its supporting holder (not shown) rather than having to replace the entire transferring unit 20. To provide the transferring roller 21 in such an arrangement that it presses against the image carrying body 10 in order to form the transfer nip when operably mounted in the image forming apparatus, it may be necessary for the transferring roller 21 to be removed for replacement along the direction of the image carrying body 10 (i.e., the direction indicated by the arrow C in FIG. 2) along which the pressing force is to be applied.
However, unfortunately, the removal of the transferring roller 21 along such direction of the image carrying body 10 is interfered by the needle shape needle shape electrode 45 if the needle shape electrode 45 is placed in the space B for the optimal charge removal performance. It is therefore difficult to arrange the needle shape electrode 45 optimally in the space B without having to remove and replace the needle shape electrode 45 together with the transferring roller 21 even if the replacement of the electrode itself may be unnecessary.